This invention relates generally to the design of anti-reflective coatings for lithography. More particularly, it relates to gradient anti-reflective coatings for lithography.
As the feature size of an integrated circuit (IC) shrinks, anti-reflective coatings (ARC) play an important role in critical dimension (CD) control (see for example Singer, xe2x80x9cAnti-Reflective Coatings: A Story of Interfaces,xe2x80x9d Semiconductor International, March 1999; Lian et al., xe2x80x9cNew Characterization Technique for SiON AR Coatings,xe2x80x9d Semiconductor International, July 1998; and Gaillard et al., xe2x80x9cPhysical and Optical Properties of an Anti-Reflective Layer Based on SiOxNy,xe2x80x9d J. Vac. Sci. Technol, Vol. A 15(5), 1997, p. 2777).
An ARC is positioned between a photoresist layer and a substrate and is designed to prevent or minimize the standing wave and swing curve effects in photolithographic patterning. The standing wave is the light intensity profile as a function of depth inside a given photoresist for a given thickness. It is the sinusoidal change due to the interference of the reflected light from the top and bottom interfaces of the resist. The swing curve is the change of clearing dose (the amount of light needed to develop the resist) as a function of resist thickness due to the standing wave effect within the resist, which varies as a function of resist thickness (see for example Cirelli et al., xe2x80x9cA Multilayer Inorganic Anti-Reflective System for Use in 248 nm Deep Ultraviolet Lithography,xe2x80x9d J. Vac. Sci. Technol. B 14(6), 1996, p. 4229). The swing curve can be measured using a stepper or it can be calculated. A typical simulation is described in Mark, xe2x80x9cAnalytical Expression for the Standing Wave Intensity in Photoresist,xe2x80x9d Appl. Optics 25, 1986, p. 1958. The effects of standing waves in ridges of patterned photoresist can also be visualized with a scanning electron microscope (SEM).
A substrate generally consists of a variety of materials, for example silicon, aluminum, polysilicon, silicon oxide, tungsten silicide, and/or copper, arranged in a mosaic type pattern.
Currently used ARCs can be classified into two types in terms of film stacks: single layer and multilayer. For a single layer ARC, the film can be organic or inorganic. Organic films work by matching the refractive index of the ARC layer with that of the resist so that there is minimal light reflected at the resist/ARC interface. These organic films are usually designed to be quite thick, e.g., 100 nm (1000 xc3x85) or more, and absorptive at the exposure wavelength, such that no light is reflected back from the ARC/substrate interface to the resist/ARC interface.
FIG. 1 is a graphic representation illustrating standing wave amplitude reduction in a conventional ARC model, having parameters shown in Table 1, at an exposure wavelength of 248 nm. A dashed curve 102 represents the standing wave pattern for a resist coated on a thick ARC (SiOxNy) layer. For the case of FIG. 1, the ARC layer is opaque at the exposure wavelength, and the ARC/Si interface does not contribute to the standing waves. Since the refractive indices of the ARC and the resist are substantially identical, the oscillation amplitude of the standing waves is correspondingly small. For comparison, a solid curve 104 represents the standing wave pattern for a resist directly coated on a Si substrate without an ARC. The ARC advantageously lowers the standing wave amplitude, but is quite thick (500 nm), adversely affecting high feature resolution.
On the other hand, inorganic ARC films are generally based on the principle of destructive interference. This is done by adjusting the optical constants (refractive index n and extinction coefficient k) and the film thickness, such that the exposure radiation that is transmitted through the resist/ARC interface and then reflected back to the resist has a similar amplitude but is 180 degrees out of phase relative to the radiation that is directly reflected from the resist/ARC interface. To achieve this condition, a single layer film must be tightly controlled in its thickness and its optical constants. SiOxNy is commonly used in an inorganic ARC. However, it is not a trivial task to find a material that can generate an exact destructive interference for a given resist on a complex substrate at a particular exposure wavelength.
To overcome this difficulty, multilayer ARCs consisting of at least two layers (typically SiOxNy) are used. Typically, the upper layer is used to generate conditions for destructive interference, whereas the lower layer is designed to absorb light at the exposure wavelength. An alternative approach is to use three or more discrete layers and to allow for gradual steps in absorption to minimize the reflection coefficient at any film interface. Multilayer ARCs are powerful on one hand, but are more complicated to process on the other hand, as precise control of multiple thicknesses and optical properties is required.
What is needed in the art of photolithography is an ARC having a small thickness and a simple structure, consisting of a minimal number of layers, and which permits maximum latitude of dimensions, optical constants, and process control parameters. Further needed is such an ARC that works effectively over substrates consisting of a variety of material surfaces and is compatible with photoresist and oxide etch processes. Also needed is such an ARC that introduces minimum strain internally between layers and externally between the ARC and resist and/or substrate.
The above needs are met by a graded anti-reflective coating (ARC) with a bottom layer that is highly absorbing at the lithographic wavelength, and with one or more layers between the bottom layer and the resist layer having inhomogeneous optical constant values through the thickness of the coating. The layer thicknesses and the values of the optical constants are preselected, such that the refractive indices at the lithographic wavelength are matched across the layer interfaces, and the optical constants vary smoothly through the layer thicknesses. In each layer the extinction coefficient and the refractive index have independently selectable values and gradients. This ARC structure provides almost total absorption in the bottom layer and near-zero reflection at the resist interface and all other intermediate interfaces of radiation at the lithographic wavelength. Preferred embodiments of the ARC contain two layers. In preferred embodiments, the optical constants of the bottom layer are homogeneously distributed, whereas in other embodiments they are smoothly varying through the bottom layer thickness. In preferred embodiments, the layers are formed of inorganic materials, typically SiOxNy. Layer thicknesses in a range from approximately 10 nm to 65 nm are typical. Some embodiments include a single absorptive layer having an extinction coefficient gradient.
Because of its highly absorbing bottom layer, an ARC according to an embodiment of the present invention works effectively over a substrate consisting of a mosaic type pattern of diverse materials, including aluminum, silicon, polysilicon, silicon oxide, tungsten silicide, and copper. It can be designed for a variety of lithographic wavelengths, including deep ultraviolet wavelengths 157 nm, 193 nm, 248 nm, and 365 nm. It provides interface reflectances of less 1.0 per cent with great latitude of manufacturing tolerances relating to layer thicknesses and optical constants.